Isobutylene is usually produced in petroleum refineries and in ethylene plants as an ingredient in a mixture of butane and butenes. Isobutylene is usually separated from the other materials by absorption in an inorganic acid such as sulfuric acid. It is then necessary to isolate isobutylene from the mixture which procedure is not only costly, but also inefficient.
The present invention is directed to the conversion of tert-butyl alkanoate to methacrolein and methacrylonitrile by treatment with an appropriate catalyst.
U.S. Pat. No. 4,065,507 discloses a process for preparing methacrolein by the oxidation of tert-butyl alcohol, alkyl tert-butyl ether, isobutylene dimer and/or isobutylene trimer with isobutylene. There is no mention of any tert-butyl esters being oxidized to methacrolein.
Tert-butyl alkanoates are known and their isolation from C.sub.4 streams containing isobutylene is also known. See U.S. Pat. No. 3,026,362, Belgian Pat. No. 819,225 and U.S. Pat. No. 4,011,272.
It is an advantage of the present process that the crude isobutylene which is isolated as a tert-butyl alkanoate can now be directly oxidized in the presence of molecular oxygen by passing the tert-butyl alkanoate over a catalyst at elevated temperatures.
This invention comprises a novel process for producing methacrolein or methacrylonitrile by the direct selective oxidation or ammoxidation of a tert-butyl alkanoate, preferably where the alkyl moiety of the alkanoate radical has from 1 to 4 carbon atoms, and includes tert-butyl formate, tert-butyl acetate, tert-butyl propionate, tert-butyl butyrate and the like.
The reaction mixture of tert-butyl alkanoate and molecular oxygen, preferably supplied from air and, optimally, including steam, is passed over a catalyst at an elevated temperature, for example, a temperature in the range of from about 200.degree. to about 600.degree. C.
Any catalyst which can be employed in the oxidation of isobutylene to methacrolein can also be employed in the oxidation or ammoxidation of tert-butyl alkanoate to methacrolein or methacrylonitrile. As examples of the type of catalysts which can be employed there are those disclosed in U.S. Pat. No. 3,907,712 which discloses catalysts of the composition having the following atomic ratios: Co:Fe:Bi:W:Mo:Si:Z is within the range of 2.0-20:0.1-10:0.1-10.0:0.5-10.0:2.0;14 11.5:0.5-15.0:0.005-1.0 wherein Z is an alkali metal; also those disclosed in U.S. Pat. No. 3,936,505 which are catalysts having the formula: Mo.sub.12 --X.sub.x --Y.sub.y --O.sub.d wherein X represents at least one of Nb and Ta: Y represents at least one of of Te, Bi, Co, W, In and Ti; x is a number from 0.1 to 9; y is a number from 0.2 to 12 and d, which is determined by the oxidation state of each component, is a number from about 36 to about 95 when each component is in the highly oxidized state; also those disclosed U.S. Pat. No. 4,035,418 which are catalysts having the formula;
Mo.sub.a Sb.sub.b Bi.sub.c Fe.sub.d Ni.sub.e Co.sub.f Sn.sub.g X.sub.h Y.sub.i O.sub.j wherein a to j represents the atomic ratio of each component and a is 12, b is 0.2 to 20; c is 0.2 to 12; d is 0.2 to 12; e is 0.2 to 12; f is 0 to 20; g is 0 to 20; h is 0.01 to 4; i is 0.01 to 4; j is a value determined by the valences of the elements in the catalysts, and X is at least one metal selected from the group consisting of potassium, rubidium, cesium, and thalium, and y is at least one metal selected from the group consisting of selenium, tellurium, gallium, vanadium, ruthenium, zinc, niobium, magnesium, chromium, manganese, cadmium, and tantalum; also U.S. Pat. No. 4,111,984, which discloses the catalyst Mo.sub.a Sb.sub.b Bi.sub.c Fe.sub.d Ni.sub.e Sn.sub.f X.sub.g Y.sub.h O.sub.i wherein X is at least one alkaline metal selected from potassium, rubidium and cesium, Y is at least one metal selected from cobalt, uranium, germanium, tungsten, and titanium, a to h are atomic ratios wherein a=12, b=0.2 to 20, c=0.2 to 12, d=0.2 to 12, e=0.2 to 12, f=0 to 20, g=0.01 to 4, h=0 to 6 and i is the value determined by the state of oxidation of the metal ion components of the catalyst; also U.S. Pat. No. 4,078,004 which discloses not only catalysts but supports for the catalysts.
In general, catalysts employed in this invention are based on oxide combinations selected from Mo-Co, Mo-Bi, Mo-Fe, Mo-P, Mo-Bi-Fe-Co which may also contain Ni, Sb, Te, W, Cs, K, Cu or combinations thereof.
The catalysts compositions described above may be prepared by the methods taught in the above-mentioned patents and also in the following U.S. Pat. Nos. 4,049,577, 3,789,063, British Pat. No. 2,023,597, U.S. Pat. No. 4,001,317.
Neither the catalyst per se nor the preparation of these catalysts form any part of the present invention. These patents are incorporated by reference to the extent necessary to enable those skilled in the art to prepare the catalyst.
In the process of the present invention, a mixture of tert-butyl alkanoate feed in vapor form and molecular oxygen (when methacrolein is desired) or molecular oxygen and ammonia, (when methacrylonitrile is desired), optionally in the presence of steam or other diluents is contacted with a catalyst as described above at an elevated temperature in the range of from about 200.degree. to 600.degree. C., for a contact time sufficient to convert the feed mainly to methacrolein or methacrylonitrile, respectively. The reactor effluent may contain methacrylic acid. The contact time may vary widely, from about 20 seconds or more. The reaction can be conducted under atmospheric, super-atmospheric, or sub-atmospheric pressures. However, in general, pressures near atmospheric, are preferred.
Any source of oxygen may be employed in the process, and, for economic reasons, it is preferred that air be employed as the source of oxygen. The mol ratio of oxygen to the hydrocarbon feed may range from between 0.5:1 to 10:1 with the preferred ratio being in the range of from about 1:1 to about 5:1.
Diluent, such as water, nitrogen, and carbon dioxide, may be present in the reaction mixture.
In general, any apparatus of the type known to be suitable for carrying out oxidation reactions in the vapor phase may be employed in the execution of this process. The process may be conducted either continuously or intermittently. The catalyst bed employed may be either a fixed-bed employing a large particulate or pelleted catalyst or a so-called "fluidized" bed of catalyst.
The reactor may be brought to the reaction temperature before or after the introduction of the reaction feed mixture. However, in a large scale operation, it is preferred to carry out the process in a continuous manner, and in such a system the recycling of any unreacted starting material is contemplated.